Air admittance valves/smells/building regs

I live in a block of 25 flats. Ground floor is commercial, 1st - 4th floors are flats.

There is no external soil stack sticking up above the roof.

The 4 'penthouse' flats each have an air admittance valve fitted on the soil pipe immediately next to the toilet pan. After some investigation, it seems these are the only soil stack ventilation for the block.

Several of the flats, including all of the penthouse flats, are experiencing intermittant drain smells. Seems to be getting slowly worse over the years and is now pretty bad. Pretty certain this is caused by u-bends having the water sucked out by low pressure.

The water level in the toilet pans of the penthouse flats is low - after flushing, within 10-20 seconds it drops to about 1" cover.

Does 4 AAV's sound sufficient for 25 flats (+ unknown usage on ground floor commercial premises)?

Is this arrangement likely comply with building regs?

Any suggestions how to remedy the problem?

Would it be worth having the AAV's replaced as they are now 9 years old (an awkward job because the cistern has been fitted immediately above, blocking easy access)?

After some research, it seems that replacing u-bends with Hep-VO's might help, but that wont be cheap.

Building is 60's-built commercial property (very high ceiling heights throughout) and converted to flats in 1996 at absolutely minimum cost; all corners cut.

Thanks and best wishes,

Mark

Does 4 AAV's sound sufficient for 25 flats (+ unknown usage on ground floor commercial premises)? Is this arrangement likely comply with building regs?

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If the conversion was done in 1996, it should have been done in accordance with building regs to the satisfaction of the local Building Control Officer. Therefore you should address your enquiry to your local council.

Personally I would be very surprised if the situation you describe was compliant. The Air Admittance Valves may well need repairing or replacing, but it would be worth having a professional look over the installation first. There may be other aspects that are sub-standard.